Bruce, the benefit of electrolysis at a depth of 1000 meters is the hydrogen arrives at the surface pressurized to 100 bar, so the transportation expense is reduced. The other problems noted are all valid but I tend to think that considering the investment the automotive sector is making in hydrogen they don't see these as insurmountable. Also with each step including electrolysis there is an added efficiency loss.

Bruce I am happy to leave to others the best way to market the energy you get from moving the heat away from the ocean's surface to the deep. I am content with the benefits derived from reducing atmospheric warming, sapping the strength of tropical storms and reducing sea level rise that you get from this process. I am intrigued though with the supergreen hydrogen technique developed by a Lawrence Livermore team, which captures carbon dioxide from the atmospheric and produces an alkaline stream that reduces the acidity of the oceans. The synergy of this process with heat pipe OTEC is pretty compelling.

I don't quite understand why if we have to stop using fossil methane it would be okay to use electrofuel methane. Perhaps you meant methanol, which I have seen suggested but here too CO2 is a byproduct of use, which it seems to me defeats the prupose?

Alistair the proposal has been presented to the Carbon War Room who claimed to be interested in reducing the carbon footprint of shipping without results. It also was presented to the Navy which it would seem would benefit for the same reasons you outline but again to no effect. They have financed some OTEC research for the purpose of producing small power systems that could service remote bases but the economics of small system doesn't work so they pulled out. To my knowledge they haven't considered fuel systems for vessels or planes over which they would have full control, which to me is a huge missed opportunity.

Buck the liquid phase is lifted by pumping not capillary action. For an 80 MW plant using ammonia as the working fluid Luis Vega estimates that 2,750 kg/s would be used. The estimated capital cost for a 100 MW plant ship is estimated at 2650 $/kw for the deep water condenser design in the MIT thesis. Locally the costs of a proposed hydro dam, the Site C, is estimated at $8.8 billion for 1,100 MW or 8000 $/kw.

Vogtle 3 and 4 for a combined 2,500 MW are estimated to cost $14 billion for a capex of about 5600 $/kw so I think the cost speaks for itself.

The most contaminated sites on the planet are Hanford and Mayak where the U.S. and Soviet Union bankrupted themselves producing plutonium. No one in their right mind is prepared to replicate that folly.